It turns out to be fairly easy. Start up the Swiki on port 8000 or what have you, get it running. Then set up Apache, which runs on port 80, thus:
<VirtualHost *:80>
DocumentRoot /somewhere
ServerName yoursitename
RewriteEngine On
RewriteRule ^/(admin|schemes)(.*)$ http://localhost:8000/$1$2 [P]
RewriteRule ^/$ http://localhost:8000/MainSwikiPage [P]
RewriteRule ^/(.+)$ http://localhost:8000/$1 [P]
</VirtualHost>